Compressed air supply systems are used in vehicles of all types, especially to supply pneumatic spring units with compressed air. Pneumatic spring units can also comprise level control devices, with which the distance between vehicle axle and vehicle body can be adjusted. A pneumatic spring unit of a pneumatic system can comprise a number of air bellows, which are pneumatically connected to a common line (gallery) and can raise the vehicle body as their filling content increases and lower the vehicle body as their compressed air content decreases. With increasing distance between vehicle axle and vehicle body, or ground clearance, the spring deflections become greater and greater, and ground unevenness can also be overcome without contact with the vehicle body occurring. Such systems are used in off-road vehicles and in sport utility vehicles and the like (SUVs). Particularly in the case of SUVs, it is desirable in the case of high performance engines to provide the vehicle, on the one hand, with comparatively small ground clearance for high speeds on the road, and, on the other hand, with a comparatively large ground clearance for off-road duty. It is also desirable to implement a change of the ground clearance as quickly as possible, which increases the requirements with regard to speed, flexibility and reliability of a compressed air supply system.
A compressed air supply system for use in a pneumatic system with a pneumatic device, for example of a pneumatic spring unit, is operated with compressed air from a compressed air feed, for example in the range of a pressure level of between about 5 and 20 bar. The compressed air is made available to the compressed air feed by means of a compressed air generator (compressor). For supplying the pneumatic device, the compressed air feed is pneumatically connected to a compressed air port and, on the other side, is pneumatically connected to a vent port. Via a vent valve arrangement, the compressed air supply system and/or pneumatic device, by releasing air, can be vented towards the vent port.
For ensuring long-term operation of the compressed air supply system, an air drier is provided, with which the compressed air is to be dried. As a result, the accumulation of moisture in the pneumatic system is avoided, which, in the case of comparatively low temperatures, can otherwise lead to valve-damaging crystallization, and to other undesirable effects in the compressed air supply system and in the pneumatic device. An air drier has a desiccant, usually a granular bed, through which the compressed air can flow so that the desiccant—at comparatively high pressure—can absorb moisture contained within the compressed air by means of adsorption. An air drier can be designed as a regenerative air drier, if applicable. To this end, the dried compressed air from the pneumatic spring system can flow through the desiccant during each venting cycle—at comparatively low pressure—in counterflow relative to the filling direction. For this, the vent valve arrangement can be opened. For such a use—also referred to as pressure swing adsorption—it has proved to be desirable to design a compressed air supply system with flexibility and at the same time reliability. In particular, a comparatively rapid venting with a pressure swing, which is nevertheless adequate for a regeneration of the air drier, is desirable.
A solenoid valve for realizing a multiple function can be achieved with at least three connected pneumatic chambers—specifically for the pneumatic connection of functionally different, always separate pneumatic chambers that are provided with different levels of pressurization—with two separate lift armatures in a common excitation coil of the solenoid valve. Each of the lift armatures is associated with a different separate pneumatic chamber in each case. In principle, such a double-armature solenoid valve is known, for example, from DE 201 60 30 or from patent cases of the present applicant, such as DE 35 01 708 A1 or DE 10 2006 041 010 A1. The basic principle of operation of a double-armature solenoid valve can be gleaned from DE 10 2004 035 763 A1, for example, which is also a case of the present applicant.
A level control device for vehicles with air filters, with which a predetermined distance of the vehicle's framework from the vehicle's axle can be maintained by filling or emptying the pneumatic springs as a function of the vehicle's load, is known from the present applicant's DE 35 429 74 A1. The device includes a safety valve that can be controlled by the pressure in the pneumatic springs.
DE 199 11 933 B4 discloses a compressed air generator with an air drier with a first compressed air supply line, wherein the compressed air is directed through a desiccant, and with a second compressed air supply line without the compressed air being directed through the desiccant.
A compressed air supply system of the general type under consideration is disclosed in EP 1 165 333 B2 within the scope of a pneumatic system with a pneumatic spring unit. In addition to a separately blockable main vent line, this has a high-pressure vent line with an additional high-pressure vent valve in addition to the main vent valve—pneumatically operated by a control valve—in the main vent line and which is connected in parallel to the main vent line. The free flow cross section of the separate high-pressure vent valve is smaller than that of the main vent valve. Such a compressed air supply system is still open to improvement. It has been shown that during the venting of such a compressed air supply system via the high-pressure vent line, the venting of dry air, which is not used for the regeneration of the desiccant, is carried out. This amounts to an unnecessary waste of dry air, especially when a flexible, fast and yet reliable operation of the compressed air supply system, which is suitable for the above applications, with a correspondingly high operating rate should be required. Via the control valve, which is provided with a comparatively small nominal diameter, a main vent valve, which can be designed with a comparatively large nominal diameter, can therefore be pneumatically pre-controlled. However, such an arrangement, which is designed for the indirect connecting of a compressed air volume, is comparatively costly.